Why switch? Why abandon a toon that had 7 million credits in the bank, Master Swordsman and Master Doctor and well on the way to becoming a Jedi?
One word answer: Sony.
Slightly longer version: The Swordsman and Doctor professions were both taken out of the game. Completely removed. As was the Jedi system. Sony had decided that the game had too much lore, was too complex and that making it simple would be great. Unfortunately, they didn't have time to test the changes before imposing them so suddenly I had a character that could not move, had no combat skills, and was called "SomethingDialog_07" This was no fun at all so I tried World of Warcraft.
WoW was amazing. My first toon was a Tauren. As I ran about the lush green hills of Mulgore, I found myself tapping the Space bar just for the fun of watching my little bull jump. The quests are no different from any other MMO. Find 10 mobs, kill them and report back for a reward. But the implementation is complete, they are challenging and the rewards are fun.
Now I've a level 60 toon, an Orc Shaman, and I am bored so no longer play except to test my http://www.craftgold.com/ program. I'm hoping the Burning Crusade catches my imagination. I've been into online games since Quake 1 and it's a bit sad not having one I am interested in.